The present invention relates to plastic articles which possess a "wet" appearance. More particularly, the present invention relates to biaxially oriented plastic articles which possess a smooth outer surface and a grainy internal surface, which internal surface is responsible for the "wet" appearance of the plastic article.
Plastic articles, and particularly bottles, are typically manufactured by a multi-step process, in which the first step is typically an injection molding process in which a thermoplastic polymer is extruded under heat and pressure to form an inflatable cylindrical preform open at one end, which is typically termed a "parison". The parison is cooled to a temperature below the crystallization temperature range of the particular polymeric resin employed, and moved to a stretch blow molding station. The parison is reheated to an elastic state and the mold dies close. A blowing medium, typically air at superatmospheric pressure, is injected into the parison interior through a nozzle or a hollow stretch stick, thereby expanding the substantially amorphous polymeric material of the parison and forcing it to assume the shape of the mold. The parison polymeric material undergoes biaxial orientation during the blow molding expansion step just described. Once expanded, the polymeric material is cooled below its glass transition temperature, blowing medium pressure is removed, the mold dies open and the newly-formed article is removed from the mold. More specific information on this method of manufacture of thermoplastic articles, and particularly plastic bottles, may be found in "Hoechst Thermoplastic PET Resin Technical Bulletin No. 2," filed concurrently herewith, the disclosure of which is hereby incorporated by reference.
M. Nishikawa et al, "Method Of Making A Frosted Bottle Of Saturated Polyester," U.S. Pat. No. 4,179,488 (Dec. 18, 1979) discloses a method for frosting saturated polyester bottles by partial crystallization which comprises heating the outer surface of a polyester parison prior to the conventional stretch blow molding expansion step. The parison outer surface is heated to induce crystallization but the parison interior is not crystallized due to the temperature gradient across the thickness of the parison cylinder wall.
I. Jackson et al, "Method And Apparatus For Inside Frosting Of Tubing," U.S. Pat. No. 4,329,314 (May 11, 1982) discloses a process for uniform frosting of the interior surface of a plastic non-biaxially oriented tube, which is not formed by an injection stretch blow molding process. The frosting effect is achieved by uniform cooling of the inner die pin of an extrusion die assembly, thereby causing micro-cracks to develop in the inner surface of the tube. The disclosure is not limited to any particular thermoplastic polymer.
J. Eustance et al, "Textured Surface Polypropylene Film," U.S. Pat. No. 4,255,381 (Mar. 10, 1981) discloses a biaxially oriented polypropylene film having a hazy, uniformly textured surface on one side, and a smooth surface on the opposite side. The textured surface consists of intertwined fibers and craters. The hazy, uniformly textured surface effect is achieved by temperature control during otherwise conventional blown tube film manufacture to markedly increase the formation of Type III spherulite crystals. The hazy, textured film so produced has utility in electrical capacitors.
Cooling of newly-formed thermoplastic articles which have been formed by the stretch blow molding process described above is typically accomplished by internal cooling of the article prior to removal of blowing medium pressure and opening of the mold dies or sections. Various patents on methods to perform such internal cooling of the newly-formed article mention internal surface defects which were avoided or minimized by practice of the disclosed method. For example, G. Trieschock et al, "Method For Blow Molding Of Thermoplastic Articles," U.S. Pat. No. 4,192,843 (Mar. 11, 1980) discloses a prior art method of cooling a newly-formed article by application of water droplets onto the interior surface of the blown article. The water droplets are non-uniformly distributed, and were alleged to mar the interior walls of the formed article. Similarly, J. Chesser, "Process Of Cooling A Blown Article," U.S. Pat. No. 3,450,805 (June 17, 1969) discusses a prior art method of cooling the interiors of newly-formed articles which comprised circulating a cooling liquid within the interior of said articles. A problem alleged with this method was too rapid cooling of the interior surfaces, which was said to result in formation of blisters or stressed and warped areas due to the non-uniform contraction of the material during the cooling operation. R. Gasmire, "Method Of And Apparatus For Making A Plastic Article," U.S. Pat. No. Re. 28,497 (originally issued Nov. 27, 1962) discloses transparent and translucent polymer bottles with pitted interior surfaces, which were manufactured by a blow molding process which employed a gaseous refrigerant as the blowing medium.
It is clear from examination of the prior art discussed above that attainment of smooth internal surfaces of injection stretch blow molded articles was a goal of prior artisans, who regarded irregular internal surfaces as defects and a problem to be solved. Likewise, it is clear the present invention is both novel and unobvious to one of ordinary skill in the pertinent art.